Researchers were surprised by the amount of bleached and dying coral they observed on a recent cruise to the Northwestern Hawaiian Islands. This June 2000 photo shows a coral reef between Lisianski Island and Midway Atoll.

Global warming
may have caused coral
bleaching, scientists say

By Diana Leone

The coral bleaching observed in the Northwestern Hawaiian Islands over the past several weeks could be caused by global warming -- or it could just be a cycle in nature, scientists say.

Either way, it offers an unprecedented opportunity to study the recovery of bleached coral reefs, said scientists who returned Tuesday from a monthlong research cruise to Hawaii's most remote islands.

The researchers said they were surprised by the amount of bleached, or dying, coral they observed there, even though they had a clue they might find bleaching because of abnormally high water temperatures.

Though the cause of the die-off and the significance of it is not known now, it is a concern because "many though not all scientists would make the linkage between increased incidences of bleaching worldwide with the general trend to global warming," said National Marine Fisheries Service marine ecologist Jean Kenyon.

"It's hard to see this, because those reefs were thriving a couple of months before that," said Rusty Brainard, a fisheries service scientist who helped lead Northwestern Hawaiian Islands Reef Assessment and Monitoring Program 2002.

Because the NOWRAMP trip was in September, just after the hottest month of August, the bleaching had just occurred, Brainard said. Though researchers have been making trips to the islands for 30 years or more, this scale of bleaching has never been observed, he said.

Bleaching refers to the startlingly white coral skeleton that is exposed when the live coral and the symbiotic algae that inhabit it are dead. If the die-off is mild, sometimes the coral can regrow. But in many cases in the world, coral bleaching has proved permanent.

The three most northwest of the islands -- Kure, Midway and Pearl & Hermes atolls -- all showed significant amounts of coral bleaching, and many of the killed coral were "decades old," Kenyon said. The bleaching seemed to be more severe in shallower water. Maro Reef and Lisianski Island, farther south, seemed to have less severe bleaching, but it extended to lower depths.

In August, water temperatures in the islands were warmer than they have been since scientists started measuring them in 1981, Brainard said. The mean August temperature has been about 83.3 degrees. This year, it spiked to 85.46 degrees.

The temperatures are actually cooler than in other locations, like the Caribbean, where coral has also been bleached, Brainard said.

It appears to mean that although the northwest corals live in cooler waters, they still are sensitive to even these slight changes in temperature, he said.

Scientists on the trip work for a variety of federal, state and education institutions. One product of their research will be more information for the Northwestern Hawaiian Islands Coral Reef Ecosystem Reserve, a temporary umbrella agency established by former President Clinton, and the planned National Marine Sanctuary that some hope to establish in its place over the next few years.

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